Do you have a tech instructor or mentor? Although some folks have their favorites, most backplates are dimensionally pretty close. If you are average build, then just find the best deal on one. Probably aluminum will serve you best for the time being.
The type of harness you get is a personal preference, unless you going the DIR route, and then you might want to make sure you have the 'approved' type. example, the type with 1 piece webbing that loops through all the points, or the 'deluxe' type with buckles and maybe even padded shoulders and loops for inflator etc.
Both those wings you linked to seem very large for your current single tank needs. For a couple of my single tank rigs, I use the HOG 32# wing and find that works pretty well with single LP72, HP100, LP104. tanks that I use regularly. If you do step up to the drysuit before you get doubles, then the 32# wing might Still work. Or you might want a bigger wing. I have a Hollis 55# wing that I use for double LP95s. That wing might actually be a little big for that, but it still works.
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